The present invention relates to a bipolar current collector separator for a fuel cell, particularly to a separator applicable to an on-vehicle fuel cell for an automobile.
Fuel cells have attracted much attention as next generation electric generators because they are excellent in energy conversion efficiency and discharge no harmful or toxic substances. In particular, fuel cells using a solid polymer electrolyte membrane (hereinafter referred to as “PEM”) that work at 150° C. or less have been studied actively and expected to be made practicable a few years later. The fuel cells using PEM can work at a relatively low temperature, be high in output density of electric generation, and be miniaturized, thereby suitable for domestic or on-vehicle fuel cells.
In general, the fuel cell using the PEM is produced by disposing the PEM between a fuel electrode and an oxygen/air electrode to prepare a cell, and by stacking a plurality of the cells with each other through plate-shaped separators having flow channels for supplying fuel gas and air. Used as the PEM is generally an ion exchange membrane of a fluororesin having a sulfonic acid group, etc., and the fuel electrode and the oxygen electrode are generally made of carbon black in which a water repellent material such as PTFE and a catalyst such as a noble metal fine particle are dispersed, etc.
When a hydrogen-oxygen fuel cell works, protons provided by oxidation of hydrogen gas penetrate into a fuel electrode (anode) and combine with water molecules therein to form H3O+, and the resultant H3O+ moves to an air electrode (cathode). In the cathode, oxygen supplied through flow channels obtains electrons provided by oxidation of hydrogen gas, and combines with protons in the electrolyte to provide water. These processes are repeated to obtain electric energy continuously. Although the theoretical electromotive force of the hydrogen-oxygen fuel cell is 1.2 V, the actual output voltage is approximately 0.6 to 0.8 V because of voltage drop due to polarization of the electrode, crossover of the reaction gas where the fuel gas leaks to the cathode through the electrolyte, contact resistance of the electrode and the collector, etc. Accordingly, to obtain the practical output voltage, it is necessary to stack dozens of cells through the separators and connect the stacked cells in series.
As known from the above-mentioned principle of electric generation, H+ exists in the electrolyte in large amounts, so that the inside of the electrolyte and the neighborhood of the electrodes where water or water vapor exists in large amounts are strongly acidic. Further, although oxygen combines with H+ to provide water in the cathode, hydrogen peroxide may be often provided depending on the working conditions. The separators are used under such circumstances, thereby required to have high chemical stability and electrochemical stability (corrosion resistance) in addition to electric conductivity and air-tightness.
Most of the conventional separators have been produced by machining a graphite plate. Although the graphite separators are low in electric resistance and high in corrosion resistance, they are poor in mechanical strength and high in machining cost. Thus, it is difficult to utilize the conventional graphite separators as the separator for the on-vehicle fuel cells, which is required to be high in the mechanical strength and low in the machining cost. Recently proposed is a separator that is produced by mixing graphite powder with a resin, and by injection-molding and baking the resulting mixture, however, there is a problem that it is low in density to be poor in the air-tightness. Although the density can be increased by impregnating the separator with a resin and by burning the resultant again to carbonize it, this results in complicated manufacturing processes. In addition, thus-produced separator has the contact electric resistance several times higher than that of the conventional graphite separator, whereby the fuel cell using this separator inevitably to the output voltage.
Metal separators have also been studied in addition to the graphite separators. The metal separators are low in bulk electric resistance, high in air-tightness and mechanical strength, and can be easily produced with reduced working cost. Further, the metal separators can be thinned and miniaturized with ease, and the weight of a fuel cell using the metal separator can be reduced if the metal separator is made of a light metal such as aluminum. However, there is a problem that the metal material is liable to corrode, in particular, it has been known that aluminum exhibits extremely high corrosion rate (R. L. Rorup et al., Mater. Res. Soc. Symp. Proc., 393 (1995), etc.). Further, there is a fear that metal ions generated by corrosion of the metal material penetrate into the electrolyte membrane to reduce the ion-conductivity thereof.
Japanese Patent Laid-Open No. 11-162478 has disclosed that the corrosion resistance of the metal separator can be improved by plating entire surfaces thereof with a noble metal. Although this metal separator plated with a noble metal sufficiently acts as a separator, it necessitates high production cost to be far from practicable. To reduce the production cost, a noble metal layer should be thinned. However, when a thinner noble metal layer is disposed by wet plating methods, the resultant layer has fine pinholes that cause corrosion of the metal separator. On the other hand, dry plating methods such as vapor deposition methods and sputtering methods are poor in efficiency of producing the thinner noble metal layer, and the resultant layer is poor in uniformity.